A Two-Stage Reaction
When you add a hydrochloric acid (HCl) solution to a solution of sodium carbonate (Na2CO3), the hydrogen ion in HCl switches places with one of the sodium ions in Na2CO3 to produce sodium hydrogencarbonate, also known as sodium bicarbonate (baking soda), and sodium chloride (salt).
Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq)
Sodium hydrogencarbonate is basic, and it reacts with the HCl still in solution to produce sodium chloride, carbon dioxide and water.
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
Phenolphthalein is a good indicator for the first reaction because it responds to the pH change caused by the formation of sodium hydrogencarbonate. It is pink in basic solutions and turns colorless as soon as the solution becomes acidic. Methyl orange, on the other hand, responds to pH changes associated with the formation of NaCl, changing from yellow to red as the solution becomes more acidic. At neutrality, it is a distinct orange color